Environmental context is critical for fire management

Guidelines need to look beyond species traits for good outcomes

Burnt heath (foreground) and woodland (background) after a large wildfire. The burnt stems in the foreground are Banksia ericifolia. This usually dominant heath plant can become locally extinct if fire intervals are too short. (Photo by Claire Foster)

Key messages:

Fire-management guidelines are often based on the traits of individual species

We studied the association between the recent fire regime and the plant community in three common vegetation types of coastal SE Australia

Guidelines for fire management of different vegetation types are often based on the traits of individual species occurring within these habitats. For example, if obligate seeding shrubs that take 5-7 years to mature occur in a particularly vegetation type, a minimum fire interval of 7 years might be prescribed (because anything less than that might see the shrub burnt before it can set seeds for the next generation, resulting in the loss of that species).

While guidelines such as these are a useful indication of fire intervals that should be avoided to prevent the loss of species, they are not so helpful in suggesting what kinds of fire regime managers should be aiming to achieve across a park or reserve. Further, this focus on individual species traits mean that vegetation types that have many species in common often have similar fire management guidelines, even if they differ substantially in structure, productivity and environmental characteristics.

An important question therefore is whether this trait-based approach to recommended fire intervals is adequate. Or should managers be aiming for different fire regimes for different vegetation types, even when those vegetation types have many species in common.

To test this question we studied the association between the recent fire regime and the plant community in three common vegetation types of coastal south-eastern Australia: dry sclerophyll forests, woodlands, and heaths. These vegetation types often occur together in complex spatial mosaics, and many species occur in multiple vegetation types. For example, in our study, 72% of the species we recorded in heath vegetation were also found in woodland sites. This similarity in species composition means that recommended fire intervals (also known as tolerable intervals) for these vegetation types are very similar: minimum intervals of greater than 7 years and maximum intervals of 30—50 years. In our study area, very few sites had experienced fire intervals outside of this range. Our study therefore tested whether, within tolerable intervals, fire regimes have different consequences for plant species richness in different vegetation types.

The effect of time since the most recent fire was consistent across vegetation types. Sites which had burnt within the last 10 years had on average 3-4 more species than sites which had not burnt for over 30 years. This pattern is common in Australia’s fire-prone ecosystems. It’s the result of short-lived plant species germinating soon after a fire, then flowering, setting seed and dying within a few years. These species may not be visible after this but they are still present, surviving the long period between fire as seeds rather than living plants. In contrast to the result for time-since-fire, we found that the effect of fire frequency on plant species richness was
completely different (in fact opposite) in different vegetation types. In dry sclerophyll woodland, sites that had burnt 4 to 6 times in 55 years contained around 15 to 20 more species than sites that had only burnt once or twice in that period.

However, in heath vegetation, sites which had burnt 4 to 6 times contained around 10-20 fewer species than sites which had burnt only twice. As these two vegetation types had nearly 70% of their species in common, this result is unlikely to be due to differences in the composition of these vegetation types.

Rather, we believe that this result has occurred because plants have responded differently to fire (and the conditions after a fire) in these different vegetation types. These differences in fire response are likely to factors such as fire intensity (how hot a fire is when it burns), competition from overstorey and midstorey species, and soil and water availability, all of which differ among vegetation types. We found support for the idea that different vegetation types can lead to different fire responses when we looked more closely at how different types of plant species were related to fire: the species that were strongly negatively related to fire frequency in heath vegetation were resprouting species (species that survive fire and regrow from storage organs), whereas this same group of species were not related to fire in woodland habitats.

What does this mean for fire management? It suggests that while using plant traits can be useful for identifying fire regimes that should be avoided, such an approach is unhelpful in predicting the outcomes of fire regimes that are within plant tolerable intervals. Because similar fire regimes can result in very different outcomes for different vegetation types, fire management plans should consider different vegetation types as separate management units, and each of these units may require different fire regimes to achieve conservation goals.

Decision Point on fire

Fire is a major structuring process of terrestrial ecosystems, especially in Australia. And managing fire is an enormously challenging and complex task, with high risk and enormous consequences for humans and nature when we get it wrong. Given this, making good decisions around our approaches to fire has always been difficult and often contested. The Decision Point archive contains many stories on fire and its management.